This invention relates to a case-hardening alloy steel and case-hardened articles made therefrom and, more particularly, to such an alloy steel which when carburized and hardened has a unique combination of surface hot hardness and temper resistance with good internal impact strength and fracture toughness.
Articles such as gears or gear trains, particularly helicopter gear systems, which require temper resistance, hot hardness, fracture toughness and impact strength for operation at elevated temperatures have been in demand to meet the more exacting operating conditions to be encountered in equipments, such as helicopters, now under development. Hitherto, such carburizing alloy steels as A.I.S.I. Type 9310, Type 3310, Type 8620 and others have been used to provide articles such as gears for such purposes. However, the more demanding operating conditions encountered in the power trains of helicopters now under development, are too rigorous for such carburizing alloy steels. For example, A.I.S.I. Type 9310 contains in weight percent:
______________________________________ C 0.08-0.13 Mn 0.45-0.65 Si 0.20-0.35 Cr 1.00-1.40 Ni 3.00-3.50 Mo 0.08-0.15 ______________________________________
With the balance iron and incidental impurities including no more than 0.025% phosphorus and 0.025% sulfur. While Type 9310 has excellent toughness, it does not have the temper resistance and hot hardness required for operation at the elevated temperatures now contemplated which may range as high as 400.degree. F. (204.degree. C.). In U.S. Pat. No. 3,713,905 granted to T. V. Philip and R. L. Vedder on January 30, 1973 and assigned to the assignee of the present application, there is disclosed an alloy steel of outstanding properties containing in weight percent:
______________________________________ Broad ______________________________________ C 0.07-0.8 Mn Up to 1 Si 0.5-2 Cr 0.5-1.5 Ni 2-5 Cu 0.65-4 Mo 0.25-1.5 V Up to 0.5 ______________________________________
With the balance iron and incidental amounts of other elements. That alloy with 0.07-0.2% carbon is readily case hardened, as by carburizing, and in that condition provides articles having good toughness and temper resistance and hot hardness better than obtainable with Type 9310. While the impact toughness of the alloy provided by that patent is high enough, its temper resistance and hot hardness are not considered to be adequate to meet such demanding conditions as those experienced by the gears in helicopters now under development. Another alloy steel which has been considered for use in making such articles as gears to be used in helicopters at temperatures up to 400.degree. F. is that disclosed in U.S. Pat. No. 3,036,912 granted to Roberts et al on May 29, 1962, but that alloy was found to have inadequate impact strength and fracture toughness.